The term “electric vehicle” used herein encompasses vehicles such as battery electric vehicles (BEV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (PHEV). A BEV includes an electric motor, wherein the energy source for the motor is a battery that is re-chargeable from an external electric grid. In a BEV, the battery is the source of energy for vehicle propulsion. A HEV includes an internal combustion engine and an electric motor, wherein the energy source for the engine is fuel and the energy source for the motor is a battery. In a HEV, the engine is the main source of energy for vehicle propulsion with the battery providing supplemental energy for vehicle propulsion (the battery buffers fuel energy and recovers kinematic energy in electric form). A PHEV is like a HEV, but the PHEV has a larger capacity battery that is rechargeable from the external electric grid. In a PHEV, the battery is the main source of energy for vehicle propulsion until the battery depletes to a low energy level at which time the PHEV operates like a HEV for vehicle propulsion.
As such, an electric vehicle has an electric motor and a battery. The motor is interposed between the battery and a drive shaft of the vehicle, wherein the motor is mechanically coupled to the driveline of the vehicle. The motor may be controlled to contribute positive wheel torque to the wheels of the vehicle in order to drive the wheels for vehicle propulsion. Conversely, the motor may be controlled to contribute negative wheel torque to the wheels in order to brake the wheels for vehicle braking.
During vehicle braking, interactions between an antilock braking system (ABS) of the vehicle, the motor, the driveline, and the road surface can result in deflections in the driveline (i.e., driveline oscillations). The driveline oscillations can cause unpleasant noise, vibration, and harshness (NVH) and can damage driveline and transmission components and/or the motor.
In particular, the motor, driveline and transmission components such as the gear box and the half shafts, and the wheels combine to create a torsional mass spring configuration. The spring configuration has a resonant frequency corresponding to the mass of the motor, the gear ratio of the transmission, and the stiffness of the wheels, the half shafts, and the gear box. The oscillation frequency of the driveline oscillations is this resonant frequency.
Active motor damping is a control algorithm for reducing driveline oscillations. An active motor damping system controls the motor to output a counter-torque to the wheels in order to damp out the driveline oscillations, particularly during an ABS operation. The active motor damping system anticipates the driveline oscillations based on the motor speed and the wheel speeds. The active motor damping system can successfully quell the driveline oscillations when the response time of the system is adequate given the oscillation frequency. However, the active motor damping system can actually worsen the driveline oscillations when the response time of the system is not adequate given the oscillation frequency.